The present invention relates to a blind stitch sewing machine with a stitch plate, a needle which swings above the stitch plate and transverse to a plunger arranged beneath the stitch plate, with a fabric feeder which moves along a substantially elliptical path from above through an opening in the stitch plate against at least one pressure foot, and in which the fabric feeder is driven from a main drive shaft over a transmission provided with an arrangement for reducing the stitch length and for stitch tacking, as well as a control device for this arrangement.
It is desirable in such blind stitch sewing machines to sew a plurality of stitches closely adjacent to each other by shortening the stitch length, which in a blind stitch sewing machine is about between 3 and 8 mm to thereby obtain a tacking of the thus-produced blind stitch seam. This is especially desirable in a single chain stitch which can be relatively easily pulled up from the rear.
A blind stitch sewing with such an arrangement for stitch tacking is for instance known from the U.S. Pat. No. 4,114,547 in which the two pressure feet can be tilted by a special adjusting device downwardly away from the stitch plate, so that the fabric feeder lacks a counter pressure for the fabric feed, thereby substantially interrupting the forward movement of the fabric, while the fabric feeder slides on the fabric. This known arrangement for stitch tacking has however essential disadvantages. The tilting away of the pressure feet in downward direction will result in improperly clamping the fabric during the stitch tacking. The fabric can therefore be displaced while the needle penetrates thereinto. This in turn will result in improperly carried-out stitch tacking, or that for instance clearly visible undesired openings in the fabric are produced during the stitch tacking. It is further possible that a heavy fabric may still be transported and no stitch tacking is produced.
Another blind stitch sewing machine of the above-mentioned kind is disclosed in the U.S. Pat. No. 4,312,290. This known blind stitch sewing machine is provided in the transmission for the drive of the fabric feeder with a device for stitch tacking by means of reducing the stitch length, including a control arrangement coordinated with this device, which comprises a plurality of cooperating element acting on the drive of the fabric feeder in such a manner that the latter is moved from its normal elliptical path in upward direction away from the stitch plate, so that the engagement of the fabric feeder with the fabric is either reduced or completely eliminated. The fabric feeder is thereby still driven from the main shaft and is moved accordingly above the stitch plate. This known construction will likewise not produce a sufficient clamping of the fabric during actuation of the device for stitch tacking, to prevent displacement of the fabric when the needle penetrates thereinto. Furthermore the additional mechanical expenditure of this known device is considerable since various additional elements are necessary in order to move the normal drive of the fabric feeder in upward direction. A further disadvantage of this known device is that an exact adjustment of the device to the respective fabric thickness is necessary.